The proposed project targets mechanisms underlying the expression of child behavioral problems classified as Attention Deficit Hyperactivity Disorder (ADHD) combined and inattentive subtypes. It builds on the applicant's recent B/START activities to initiate in depth investigation of key areas. ADHD represents a major public health problem. Children with this syndrome pose vexing problems for parents, schools, and the criminal justice system, place extensive demands on mental health services nation-wide, and are at risk for a range of problematic outcomes in adolescence and adulthood. Conceptualizing ADHD as reflecting a core inhibitory dysfunction developing in interaction with family context, the proposed work has two aspects. First, it aims to better define the nature of inhibitory deficit in ADHD by applying current neuropsychological and cognitive models to family marker studies (i.e., considering performance in ADHD children and first degree relatives). Secondly, it aims to begin exploratory integration of child neuropsychological deficits with selected aspects of the family context. Families will be recruited via a population based multi-stage screening procedure and will then complete neuropsychological executive tasks. The procedures entail examining different kinds of inhibitory processes, using tasks from cognitive psychology already identified with ADHD (such as the stop-signal paradigm) as well as inhibitory tasks not typically used in ADHD (e.g., negative priming tasks). These tasks operationalize a model of inhibition as including dissociable functions for behavioral inhibition, cognitive inhibition, and interference control. It is posited that this model can characterize the respective cognitive profiles of the ADHD subtypes in a double dissociation manner. By joining neuropsychological factors with selected aspects of family context (e.g., parent personality and psychopathology, parental social support) the work develops an empirically based transactional systems account of the expression and maintenance of ADHD behavioral symptoms. All of these objectives in turn can shed light on nosology and etiological mechanisms, thus contributing to better evaluation and interventions by identifying which children are at risk, and which contextual factors may combine to activate the risk factors for these children.